Modelling marine stratocumulus and its radiative properties

The radiative properties of sub-tropical marine stratocumulus, which cover large regions and affect the global climate, are influenced by the presence of drizzle, aerosols, and cloud-top entrainment, all of which are coupled. VOCALS, the VAMOS Ocean-Cloud-Atmosphere-Land Study, is examining the climate system of the southeast Pacific to reduce uncertainties in current and future climate projections, especially those associated with marine stratocumulus and coupled ocean-atmosphere processes. As part of VOCALS-UK we are investigating the small-scale structure and microphysics of marine stratocumulus, and hence its radiative properties, by using the UK Met Office Large Eddy Model (LEM). The model simulations are validated against measurements from the BAe-146 research aircraft obtained during the VOCALS field campaign over the southeast Pacific in October and November 2008.

We have incorporated the new Morrison microphysics scheme into the LEM and compared simulations with the Morrison scheme to those from the LEM standard bulk microphysics. Simulations with the Morrison scheme tend to produce a deeper marine stratocumulus with increased Liquid Water Path (LWP), but reduced precipitation compared to the standard LEM scheme. The cloud droplet effective radii (Re) is increased as not all the cloud condensation nuclei (CCN) are activated, so that radiative cooling at the cloud tops, vertical mixing and cloud top entrainment are all reduced. Overall, results from the Morrison scheme are in better agreement with the measurements.

We then carried out sensitivity studies to quantify the effects of increased CCN number concentration, entrainment of clean air at cloud top, and precipitation on the simulated clouds and their radiative properties, including the inhomogenity over different domain sizes. Increased CCN results in reduced Re, increased albedo, cloud top cooling, vertical mixing and entrainment, and reduced LWP, inhomogeneity and precipitation. Entrainment of clean air results in slightly increased Re, reduced albedo, cooling, vertical mixing, entrainment and LWP, and increased inhomogeneity and precipitation. Precipitation removes liquid water from the cloud, but evapouration of the raindrops then moistens and cools the air below, reducing LWP, vertical mixing and albedo, and increasing cloud layer inhomogeneity. Furthermore the effects are coupled with increased CCN leading to reduced precipitation, and precipitation removing CCN.

The amount of horizontal inhomogeneity in the albedo of the simulated cloud tops over different domain sizes is also examined. Increasing CCN reduces the inhomogeneity across all domain sizes, while both the entrainment of clean air and precipitation increase the inhomogeneity with the greatest increases over the largest domains.